(a) Technical Field
The present invention relates to a method for recovering the performance of a fuel cell stack. More particularly, the present invention relates to a method for directly recovering the deteriorated performance of a fuel cell stack mounted within a vehicle without detachment of the fuel cell stack.
(b) Background Art
A fuel cell stack is used in a fuel cell vehicle as a main power source and is formed of tens to hundreds of stacked unit cells. FIG. 2 schematically shows an exemplary basic configuration of a membrane electrode assembly (MEA) in a unit cell of a fuel cell stack. As shown in the figure, the membrane electrode assembly is disposed at the innermost side of the unit cell of the fuel cell stack.
The membrane electrode assembly includes a polymer electrolyte membrane 10 that conducts protons and an anode 14 and a cathode 12 as electrodes stacked on either side of the electrolyte membrane to allow hydrogen as a fuel gas and oxygen as an oxidizing gas to react. The anode 14 and the cathode 12 are formed of platinum (Pt), a catalytic material, supported on carbon, i.e. a Pt/C catalyst electrode layer.
Although not shown in FIG. 1, a gas diffusion layer (GDL), a gasket, etc., are stacked extraneous to the cathode 12 and the anode 14, and a separator is stacked extraneous to the gas diffusion layer. The separator provides flow channels that supply reaction gases, i.e. hydrogen and oxygen (or oxygen-containing air) and allow discharge of water produced from reaction and passage of cooling water. Additionally, an end plate that supports and fixes the outermost unit cell, a connector, etc. are joined outside the cells to complete a fuel cell stack.
At the anode of the fuel cell stack, oxidation of hydrogen occurs as described in the following reaction formula, outputting protons and electrons. The generated protons and electrons move toward the cathode through the polymer electrolyte membrane and the separator, respectively. At the cathode, the protons which moved from the anode react with electrons and oxygen included in air to form water. Electric energy is produced from the fuel cell stack as the electrons travel.
—Electrode Reactions—Anode: hydrogen oxidation 2H2→4H++4e−Cathode: oxygen reduction 4H++4e−+O2→2H2OTotal: 2H2+O2→2H2O
As the fuel cell stack is operated, deterioration occurs at the polymer electrolyte membrane and the catalytic electrodes (Pt/C), i.e., the cathode and the anode, of the membrane electrode assembly (MEA). As a result, the performance of the fuel cell stack (i.e., stack output) decreases with time. In particular, when an oxide film (e.g., Pt—OH, Pt—O, etc.) is formed on the surface of the platinum (Pt) cathode due to, for example, deterioration, the oxide film interferes with the adsorption of reactive oxygen (O2) on the platinum surface, thus slowing the oxygen reduction reaction (ORR) on the cathode and resulting in decreased stack performance.
Furthermore, Pt cations (Ptz+) released from the oxide on the platinum surface during the operation may redeposit on the surface of other platinum particles, leading to increased size of the platinum particles. In addition, corrosion of carbon occurring during the operation results in a weaker binding between platinum and carbon, leading to aggregation of platinum particles of nanometers in size. The increased size of the platinum particles causes decreased catalytic activity. However, the decrease in stack performance caused by chemical change on the surface of the platinum catalyst is recognized as irreversible deterioration and there are few studies or reports on a method for recovering (e.g., rehabilitating) the performance of the membrane electrode assembly.
According to a method of a related application, a procedure of supplying hydrogen to a cathode of a deteriorated fuel cell stack is repeated at least 3 times and the hydrogen is also stored for a predetermined time to remove the oxide formed on the platinum surface of the cathode. In particular, by repeating 3 times a procedure of supplying hydrogen at about 70° C. to a cathode 12 (see FIG. 1) of a deteriorated fuel cell for 1 hour and storing the same for a day, the oxide film (Pt—OH, Pt—O, etc.) formed on the platinum (Pt) surface of the cathode 12 can be removed and, at the same time, mobile platinum ions (mobile Ptz+, z=2 or 4) released during the operation of the fuel cell can be redeposited as platinum (Pt) with high activity through recombination with electrons. As a result, the catalytic activity of the cathode can be recovered and, through this, the stack performance can be recovered by about 30-40%.
Since the hydrogen supplied to the cathode 12 for 1 hour is diffused to an anode 14 (indicated by a broken arrow in FIG. 1), hydrogen atmosphere can be created at both electrodes and the catalyst oxide at the cathode can be reduced. However, this method requires an increased recovery time of the performance of the fuel cell and an excessive amount of hydrogen may be supplied to the cathode. For these reasons, it may be difficult to recover the performance of the fuel cell stack without detachment from a fuel cell vehicle.